The present invention relates to an improved delignifying method for wood pulp and more specifically to an improved method for gas phase delignifying wood pulp using chlorine dioxide.
Chlorine dioxide has found wide use as a disinfectant in water treatment/purification, as a bleaching agent in pulp and paper production, and a number of other uses because of its high oxidizing power. There are a number of chlorine dioxide generator systems and processes available in the marketplace. Most of the very large scale generators utilize a chlorate salt, a chloride ion source or reducing agent, and a strong acid. In the presence of chloride ion and acid, chlorate ion reacts to produce a mixture of chlorine and chlorine dioxide. The chlorine present is an undesired by-product.
Many processes have been developed to produce chlorine dioxide with lower chlorine concentrations by adding a reducing agent.
Reducing agents which have been used include methanol or other organic compounds, sulfur, sulfur dioxide or other sulfur-oxygen species having a sulfur valence of less than +6, and carbon monoxide among others. When organic compounds are used, unreacted volatile organics including organic acids are present in the product gas. Using sulfur containing reducing agents, the sulfate or sulfuric acid produced accumulates as a waste product. When gaseous reducing agents such as sulfur dioxide or carbon monoxide are employed, reactor designs and process control systems must protect against unreacted reducing agent leaving the system with the chlorine dioxide gas.
In addition, prior art processes for the production of chlorine dioxide from chlorate salts require an excess of the acid used. This acid is slowly neutralized by the accumulation of alkali metal ions that enter the process with the chlorate salt. The accumulation of salts must be removed as a waste stream, either liquid or solid, in every process currently practiced commercially.
To avoid the formation of an acidic alkali metal salt, it has been proposed that chlorine dioxide be prepared from chloric acid. Chloric acid is, however, not commercially available. Its preparation has been taught, for example, in U.S. Pat. No. 3,810,969 issued May 14, 1974 to A. A. Schlumberger. Schlumberger teaches a process for producing chloric acid by passing an aqueous solution containing from 0.2 gram mole to 11 gram moles per liter of an alkali metal chlorate such as sodium chlorate through a selected cationic exchange resin at a temperature from 5.degree. to 40.degree. C. The process produces an aqueous solution containing from 0.2 gram mole to about 4.0 gram moles of HClO.sub.3 per liter.
K. L. Hardee et al, in U.S. Pat. No. 4,798,715 issued Jan. 17, 1989, describe a process for chlorine dioxide which electrolyzes a chloric acid solution produced by passing an aqueous solution of an alkali metal chlorate through an ion exchange resin. The electrolysis is carried out using an electrocatalytic cathode where the catalyst is, for example, one or more valve metal oxides which may be combined with a platinum group metal oxide, or a platinum group metal, or oxides of a platinum group metal, magnetite, ferrite, or mixed metal oxides.
The electrolyzed solution contains a mixture of chlorine dioxide, and chloric acid which is fed to an extractor in which the chlorine dioxide is stripped off. The ion exchange resin is regenerated with hydrochloric acid and an acidic solution of an alkali metal chloride formed.
Processes which produce chloric acid in an ion exchange resin require the regeneration of the ion exchange resin with acid to remove the alkali metal ions and the use or treatment and disposal of the acidic salt solution. In addition, the concentration of chloric acid which can be produced by an ion exchange process is limited as more concentrated chloric acid solutions attack the ion exchange resins used in the process. Further, the production of chloric acid by means of a cation exchange resin has not been economically attractive.
Gas phase bleaching with chlorine dioxide has been proposed over the years in various patents and publications as a method of reducing the bleaching time while cutting chemical costs. Gas phase bleaching is carried out on higher consistency pulp using mixtures of chlorine dioxide and steam and/or inert gases such as air or nitrogen.
U.S. Pat. No. 3,725,193, issued Apr. 3, 1973 to R. M. DeMontigny et al., describes a process for bleaching high consistency pulps, which includes preheating the pulp by direct steaming. A gaseous mixture of chlorine dioxide diluted with steam or a non-reactive gas is then passed through the pulp. The contact period is in the order of a fraction of a second. The bleached pulp was then held in a retention vessel for 30 minutes. Unreacted chlorine dioxide was removed from a bleaching tower by aeration. The final pH of the bleached pulp was 5.2.
Advantages alleged for gas phase bleaching of high consistency wood pulps include superior control of bleaching because of the short retention times employed; reduced chemical usage for the same brightness; and low water usage and effluent volume among others. Progress in achieving these advantages for gas phase bleaching commercially have been thwarted by the lack of a process for generating chlorine dioxide gas instantaneously, that is, a process having inconsequential start-up times and shut-down times so that the chlorine dioxide gas can be used directly from the generator without requiring the formation and storage of dilute aqueous solutions of chlorine dioxide which are subsequently stripped. In addition, previous processes for bleaching with chlorine dioxide gas have used chlorine dioxide having significant concentrations of chlorine gas which result in undesired damage to the cellulose.